1. Field of Invention
The present invention relates to a heart-sound detecting apparatus and particularly to such an apparatus which detects a heart sound by extracting or determining, from a physical signal containing a heart-sound component, the heart-sound component.
2. Description of Related Art
Heart sounds are detected to make diagnosis of heart disease. In addition, heart sounds may be detected to determine, in combination with a pulse wave, a pulse-wave propagation velocity at which the pulse wave propagates through a living subject. In addition, since pulse-wave propagation velocity reflects arteriosclerosis which, in turn, directly relates to hypertension, it is desirable to measure, the velocity as part of a daily medical check at home.
In general, heart sounds are detected using a heart-sound microphone which is usually directly worn on the skin of a prescribed portion of the chest, e.g., right above the heart. A heart-sound signal provided by the heart-sound microphone worn on the chest has a great signal-to-noise (S/N) ratio. Thus, the heart-sound signal which is filtered by a standardized filter can directly be used to determine a reference point for making diagnosis or determining pulse-wave propagation velocity.
However, when the heart-sound microphone is used to detect heart sounds, the subject needs to take off their clothes and thereby expose the chest. Thus, it is considerably cumbersome to detect heart sounds using the heart-sound microphone. In particular, when pulse-wave propagation velocity is measured at home as part of a daily medical check, it is desirable to detect heart sounds in an easier manner.
In this background, it has been proposed to provide a heart-sound detecting apparatus which detects a heart sound at a position distant from the chest of a living subject, based on the fact that the heart sound is transmitted from the heart to that position. This apparatus is disclosed in U.S. Patent Application claming priority from Japanese Patent Application No. 2001-030879. This apparatus includes a pressure-pulse-wave sensor which is worn on an upper arm of a subject and extracts a heart-sound component from a pressure-pulse-wave signal supplied from the sensor.
However, the magnitude of the heart-sound component contained in the pressure-pulse-wave signal or physical signal detected from the upper arm distant from the chest is weak. The magnitude of the other signal components, such as physical sounds occurring to the upper arm, external noise, or artifact, is also strong. Therefore, in many cases, the signal filtered by the conventional, standardized filter may not be used to determine one or more reference points which are needed to make diagnosis or determine pulse-wave propagation velocity.
It is therefore an object of the present invention to provide a heart-sound detecting apparatus which can accurately determine a heart-sound component which is contained in a physical signal, even if the magnitude of the heart-sound component may be weak relative to the magnitude of the physical signal as a whole.
The Inventors have intensively studied about how to achieve the above-indicated object, and obtained the following findings. Usually, a heart-sound signal supplied by a heart-sound microphone is filtered by a filter having a considerably large frequency range. The heart-sound signal is filtered because there are great differences among respective heart-sound signals obtained from individual subjects. That is, the respective heart sounds of individual subjects may have largely different frequency ranges and/or waveforms. Hence, the Inventors have found that if a heart-sound frequency range and/or a heart-sound waveform of each individual subject are/is determined in advance as heart-sound characteristic information, a heart-sound component can be accurately determined based on the heart-sound characteristic information. The heart-sound component can be accurately determined from a physical signal actually detected from the each subject even if the magnitude of the heart-sound component may be weak relative to that of the physical signal as a whole. The present invention has been developed based on this finding.
The above object has been achieved by the present invention. According to a first feature of the present invention, there is provided an apparatus for detecting a heart sound of a living subject, comprising a memory device which stores heart-sound characteristic information which is characteristic of a heart sound of the subject, a pressure-pulse-wave sensor which is adapted to be worn on a limb of the subject, detects a pressure pulse wave which is produced from an artery of the limb and is propagated from the artery to the pressure-pulse-wave sensor, and produces a pressure-pulse-wave signal representing the detected pressure pulse wave and containing a heart-sound component; and a heart-sound determining means for determining, based on the heart-sound characteristic information stored in the memory device, the heart-sound component contained in the pressure-pulse-wave signal.
According to this feature, the memory device stores heart-sound characteristic information which is characteristic of a heart sound of the living subject, and the heart-sound determining means determines, based on the heart-sound characteristic information characteristic of the heart sound, the heart-sound component contained in the pressure-pulse-wave signal supplied by the pressure-pulse-wave sensor. Therefore, even if the magnitude of the heart-sound component may be weak relative to that of the physical signal as a whole, the present apparatus can accurately determine the heart-sound component.
According to a second feature of the present invention, the heart-sound characteristic information comprises a heart-sound microphone which is adapted to be worn on the chest of the subject and detects, in advance, the heart sound of the subject; and a heart-sound characteristic-information obtaining means for obtaining the heart-sound characteristic information from the heart sound detected in advance by the heart-sound microphone from the chest of the subject. The heart-sound characteristic-information obtaining means obtains the heart-sound characteristic information comprising a heart-sound frequency range consisting of a plurality of frequencies which are predetermined by subjecting, to a frequency analysis, the heart sound detected in advance by the heart-sound microphone from the chest of the subject. The heart-sound determining means comprises a first heart-sound determining means for extracting from the pressure-pulse-wave signal, the heart-sound component having the plurality of frequencies of the heart-sound frequency range.
According to this feature, the memory device stores the heart-sound frequency range which is predetermined based on the heart sound detected in advance from the chest of the subject and which is characteristic of the subject, and the heart-sound determining means extracts, from the pressure-pulse-wave signal supplied by the pressure-pulse-wave sensor, the heart-sound component having the frequencies of the heart-sound frequency range. Thus, even if the magnitude of the heart-sound component may be weak relative to that of the physical signal as a whole, the present apparatus can accurately extract the heart-sound component.
According to a third feature of the present invention, the heart-sound characteristic information comprises a heart-sound microphone which is adapted to be worn on the chest of the subject and detects, in advance, the heart sound of the subject, and a heart-sound-characteristic-information obtaining means for obtaining the heart-sound characteristic information from the heart sound detected in advance by the heart-sound microphone from the chest of the subject. The heart-sound-characteristic-information obtaining means obtains the heart-sound characteristic information comprising a first portion of the heart sound detected in advance by the heart-sound microphone from the chest of the subject, said first portion being detected during a predetermined time interval. The heart-sound determining means comprises a second heart-sound determining means for determining as the heart-sound component, a second portion of the pressure-pulse-wave signal supplied by the pressure-pulse-wave sensor, said second portion having a length corresponding to the predetermined time interval and having a waveform best approximating a waveform of said first portion of the heart sound.
According to this feature, the memory device stores, as the heart-sound, characteristic information characteristic of the subject, a portion of the heart sound which is detected in advance from the chest of the subject and which has a great signal-to-noise ratio. Since the heart-sound determining means determines, as the heart-sound component, a portion of the pressure-pulse-wave signal detected by the pressure-pulse-wave sensor that has a length corresponding to the predetermined time interval and has a waveform best approximating a waveform of the portion of the heart-sound signal stored in the memory device, the present apparatus can accurately determine the heart-sound component from the pressure-pulse-wave signal even if the magnitude of the heart-sound component contained in the pressure-pulse-wave signal may be weak.
According to a fourth feature of the present invention, the heart-sound characteristic information comprises a heart-sound microphone which is adapted to be worn on the chest of the subject and detects, in advance, the heart sound of the subject; and a heart-sound-characteristic-information obtaining means for obtaining the heart-sound characteristic information from the heart sound detected in advance by the heart-sound microphone from the chest of the subject. The heart-sound-characteristic-information obtaining means comprises a frequency-time analyzing means for subjecting, to a frequency-time analysis, the heart sound detected in advance by the heart-sound microphone from the chest of the subject, and thereby providing a frequency-time analyzed signal. The heart-sound-characteristic-information obtaining means obtains the heart-sound characteristic information comprising a first portion of the heart sound detected in advance by the heart-sound microphone from the chest of the subject, said first portion having a plurality of frequencies of a heart-sound frequency range which is predetermined based on the frequency-time analyzed signal provided by the frequency-time analyzing means, and being detected during a predetermined time interval. The heart-sound determining means comprises a first heart-sound determining means for extracting, from the pressure-pulse-wave signal, a signal component having the plurality of frequencies of the heart-sound frequency range; and a second heart-sound determining means for determining, as the heart-sound component, a second portion of the signal component extracted by the first heart-sound determining means, said second portion having the plurality of frequencies of the heart-sound frequency range, having a length corresponding to the predetermined time interval, and having a waveform best approximating a waveform of said first portion of the heart sound.
According to this feature, the memory device stores a portion of the heart sound, detected in advance from the chest of the subject, that has the frequencies of a heart-sound frequency range which is predetermined based on a frequency-time analyzed signal of the heart sound, and is detected during a predetermined time interval. The heart-sound determining means determines; as the heart-sound component, a portion of the signal component extracted by the first heart-sound determining means that has the frequencies of the heart-sound frequency range, has a length corresponding to the predetermined time interval, and has a waveform best approximating a waveform of the stored portion of the heart sound. Thus, the present apparatus can accurately determine the heart-sound component from the physical signal.